1. The Field of the Invention
The present invention relates to compositions and methods for manufacturing molded sheets and articles therefrom. More particularly the present invention relates to sheets having a starch-bound matrix reinforced with fibers and optionally including an inorganic mineral filler. The molded sheets may be substituted for conventional paper and paperboard products.
2. The Relevant Technology
A. Sheets, Containers, and Other Articles
Thin, flexible sheets made from materials such as paper, paperboard, plastic, polystyrene, and even metals are presently used in enormous quantity as printed materials, labels, mats, and in the manufacture of other articles such as containers, separators, dividers, envelopes, lids, tops, cans, and other packaging materials. Advanced processing and packaging techniques presently allow an enormous variety of liquid and solid goods to be stored, packaged, or shipped while being protected from harmful elements.
Containers and other packaging materials protect goods from environmental influences and distribution damage, particularly from chemical and physical influences. Packaging helps protect an enormous variety of goods from gases, moisture, light, microorganisms, vermin, physical shock, crushing forces, vibration, leaking, or spilling. Some packaging materials also provide a medium for the dissemination of information to the consumer, such as the origin of manufacture, contents, advertising, instructions, brand identification, and pricing.
Typically, most containers and cups (including disposable containers) are made from paper, paperboard, plastic, polystyrene, glass and metal materials. Each year over 100 billion aluminum cans, billions of glass bottles and thousands of tons of paper and plastic are used in storing and dispensing soft drinks, juices, processed foods, grains, beer, etc. Outside of the food and beverage industry, packaging containers (and especially disposable containers) made from such materials are ubiquitous. Paper for printing, writing, and photocopying, as well as magazines, newspapers, books, wrappers, and other flat items made primarily from tree derived paper sheets are also manufactured each year in enormous quantities. In the United States alone, approximately 5 1/2 million tons of paper are consumed each year for packaging purposes, which represents only about 15% of the total annual domestic paper production.
B. The Impact of Paper, Plastic, Glass and Metal
Recently there has been a debate as to which of these materials (e.g., paper, paperboard, plastic, polystyrene, glass, or metal) is most damaging to the environment. Consciousness-raising organizations have convinced many people to substitute one material for another in order to be more environmentally "correct." The debate often misses the point that each of these materials has its own unique environmental weaknesses. One material may appear superior to another when viewed in light of a particular environmental problem, while ignoring different, often larger, problems associated with the supposedly preferred material. In fact, paper, paperboard, plastic, polystyrene, glass, and metal materials each have their own unique environmental weaknesses.
Polystyrene products have more recently attracted the ire of environmental groups, particularly containers and other packaging materials. While polystyrene itself is a relatively inert substance, its manufacture involves the use of a variety of hazardous chemicals and starting materials. Unpolymerized styrene is very reactive and therefore presents a health problem to those who must handle it. Because styrene is manufactured from benzene (a known mutagen and probably a carcinogen), residual quantities of benzene can be found in styrene. Finally, because polymerized styrene is relatively stable under ordinary conditions, containers, packing peanuts, and other articles made therefrom resist breakdown and therefore persist over long periods of time when discarded into the environment.
More potentially damaging has been the use of chloro-fluorocarbons (or "CFCs") in the manufacture of "blown" or "expanded" polystyrene products. This is because CFCs have been linked to the destruction of the ozone layer. In the manufacture of foams, including blown polystyrene, CFCs (which are highly volatile liquids) have been used to "expand" or "blow" the polystyrene into a foamed material, which is then molded into the form of cups, plates, trays, boxes, "clam-shell" containers, spacers, or packaging materials. Even the substitution of less "environmentally damaging" blowing agents (e.g., HCFC, CO.sub.2, and pentanes) is still significantly harmful and their elimination would be beneficial.
As a result, there has been widespread pressure for companies to stop using polystyrene products in favor of more environmentally safe materials. Some environmental groups have favored a temporary return to the use of more "natural" products such as paper or other products made from wood pulp, which are believed to be biodegradable. Nevertheless, other environmental groups have taken the opposite view in order to minimize the cutting of trees and depletion of forests.
Although paper products are ostensibly biodegradable and have not been linked to the destruction of the ozone layer, recent studies have shown that the manufacture of paper probably more strongly impacts the environment than does the manufacture of polystyrene. In fact, the wood pulp and paper industry has been identified as one of the five top polluters in the United States. For instance, products made from paper require ten times as much steam, fourteen to twenty times as much electricity, and twice as much cooling water compared to an equivalent polystyrene product. Various studies have shown that the effluent from paper manufacturing contains ten to one hundred times the amount of contaminants produced in the manufacture of polystyrene foam.
Another drawback of the manufacture of paper and paperboard is the relatively large amount of energy that is required to produce paper. This includes the energy required to process wood pulp to the point that the fibers are sufficiently delignified and frayed such that the fibers are essentially self-binding under the principles of web physics. In addition, a large amount of energy is required in order to remove the water within conventional paper slurries, which contain water in amounts of up to about 99.5% by volume. Because so much water must be removed from the slurry, it is necessary to literally suck water out of the slurry even before heated rollers can be used to dry the sheet. Moreover, much of the water that is sucked out of the sheets during the dewatering process is usually discarded into the environment.
The manufacturing processes of forming metal sheets into containers (particularly cans made of aluminum and tin), blowing glass bottles, and shaping ceramic containers utilize high amounts of energy because of the necessity to melt and then separately work and shape the raw material into an intermediate or final product. These high energy and processing requirements not only utilize valuable energy resources, but they also result in significant air, water, and heat pollution to the environment. While glass can be recycled, that portion that ends up in landfills is essentially non-degradable. Broken glass shards are very dangerous and can persist for years.
Even paper or paperboard, believed by many to be biodegradable, can persist for years, even decades, within landfills shielded from air, light, and water, all of which are required for normal biodegradation activities. There are reports of telephone books and newspapers having been lifted from garbage dumps that had been buried for decades. This longevity of paper is further complicated since it is common to treat, coat, or impregnate paper with various protective materials which further slow or prevent degradation.
Another problem with paper, paperboard, polystyrene, and plastic is that each of these requires relatively expensive organic starting materials, some of which are nonrenewable, such as the use of petroleum in the manufacture of polystyrene and plastic. Although trees used in making paper and paperboard are renewable in the strict sense of the word, their large land requirements and rapid depletion in certain areas of the world undermines this notion. Hence, the use of huge amounts of essentially nonrenewable starting materials in making sheets and articles therefrom cannot be sustained and is not wise from a long term perspective. Furthermore, the processes used to make the packaging stock raw materials (such as paper pulp, styrene, or metal sheets) are very energy intensive, cause major amounts of water and air pollution, and require significant capital requirements.
In light of the foregoing, the debate should not be directed to which of these materials is more or less harmfull to the environment, but rather toward asking whether an alternative material can be developed which will solve most, if not all, of the various environmental problems associated with each of these presently used materials.
C. Starch Binders.
More recently, many have attempted to utilize starches and starch derivatives as the binding agent or sole constituent within molded articles. One method for molding starch is by means of forming what is known in the art as "destructurized starch." In the manufacture of destructurized starch, native starch or starch derivatives are mixed with a plasticizing agent and liquified under high temperature and pressure in order to create a "hot melt" which is solidified by cooling the hot melt to below the "glass transition temperature." In this way, starch is treated like a thermoplastic material. While destructurized or hot melt starch systems sound easy in theory, in practice the manufacturing methods are quite expensive and the articles made therefrom are generally unsatisfactory and of low quality.
Another method for molding starch-based mixtures into articles involves molding an aqueous starch mixture between heated dies. The starch binder is preferably initially in an unmodified, ungelatinized state within the moldable aqueous mixture. Otherwise, the mixture would have to include far more water in order to maintain the same characteristics of moldability due to the gelation of starch and the tremendous viscosity increasing effect of gelatinized starch within water. The starch water mixtures are heated between the molds to a temperature great enough to gelatinize the starch as well as to remove the majority of the water from the moldable mixture. The resulting molded articles can be demolded, but are initially very brittle until they have been "conditioned" by placing them in a high humidity chamber for extended periods of time in order to reabsorb moisture.
Simply demolding the articles to have residual moisture has not proven feasible due to the tendency of the foamed cellular starch matrix to collapse if not sufficiently dried and hardened. However, obtaining a cellular starch matrix having sufficient strength to avoid collapse usually entails overdrying the starch. Such a conditioning is a required after molding process. While the foregoing molding process may have some utility, it does not allow for the continuous manufacture of continuous sheets, such as in conventional paper making processes.
Starch derivatives are also widely used in the paper industry as sizing agents and coatings in order to seal the pores of paper and create a smoother, less porous surface. However, conventional paper manufacturing processes universally rely on the principle of web physics, which is the intertwining of and hydrogen bonding between fibers, in order to form the bonding matrix of the sheet. The starch binders added to the paper slurry or furnish only act as secondary binding agents since most of the starch will pass through the forming wire along with the water as it is drained from the furnish during the paper-making process. Hence, much of the starch added to paper furnish is wasted. It therefore would be highly uneconomical to utilize starch as the sole or primary binder in conventional paper.
Moreover, one of the problems with starch binders is that they are generally very sticky once dissolved or gelatinized in water. While this makes them generally good binding agents, it complicates the manufacturing process since sheets or articles made using large amounts of dissolved or gelated starch binders have a tendency to stick to the mold or sheet-forming apparatus. On the other hand, unmodified starch granules are generally insoluble in water and merely act as passive particulate fillers in wet systems unless the compositions containing starch granules are heated to above the gelation temperature of the starch. However, once gelated, the unmodified starch granules will, of course, become very sticky and tend to adhere to the molding equipment, particularly heated molding equipment.
Based on the foregoing, what is needed are improved compositions and methods for manufacturing low cost, environmentally friendly sheets which had properties similar to paper, paperboard, polystyrene, plastic, or metal sheets.
It would be a significant improvement in the art if such sheets could be formed into a variety of containers or other articles using existing manufacturing equipment and techniques presently used to form articles from paper, paperboard, polystyrene, plastic, or metal sheets.
It would yet be an advancement in sheet-making if the environmentally friendly sheets could be formed from molding compositions which contain only a fraction of the water and/or fibers contained in typical slurries used to make conventional paper and which did not require extensive dewatering during the sheet forming process.
It would be a significant improvement in the art if such sheets, as well as containers or other articles made therefrom, were readily biodegradable and/or degradable into substances commonly found in the earth.
From a practical point of view, it would be a significant improvement to provide compositions and methods which allowed the manufacture of sheets, containers, and other articles therefrom at a cost that was comparable to or even lower than the cost of existing methods of manufacturing paper, plastics, or metal products. Specifically, it would be desirable to reduce the energy requirements and initial capital investment costs for making products having the desirable characteristics of paper, plastics, or metals.
It would be a further advancement in the art to provide compositions and methods which allowed for the inclusion of relatively high amounts of starch within sheets while overcoming the problems associated with the adhesion of starch, particularly gelatinized starch, to the molding or sheet forming apparatus.
It would also be a tremendous advancement in the art to provide compositions and methods which allowed for the optional inclusion of significant quantities of natural inorganic mineral fillers within the aforementioned sheets. In particular, it would be a significant improvement in the art if such inorganically filled sheets had greater flexibility, tensile strength, toughness, moldability, and mass-producibility compared to prior materials having a high content of inorganic filler.
Such compositions and methods for manufacturing the aforementioned sheets are disclosed and claimed herein.